Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes.
When a fluid working machine operates as a pump, a low pressure manifold typically acts as a net source of fluid and a high pressure manifold typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold typically acts as a net source of fluid and a low pressure manifold typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” are relative, with the relative pressures being determined by the application. A fluid working machine may have more than one low pressure manifold and more than one high pressure manifold. First and second manifolds may operate as low and high pressure manifolds or high and low pressure manifolds respectively in alternative operating modes.
Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could alternatively be a gas or a compressible liquid.
Fluid working machines are known which comprise a plurality of working chambers of cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis and in phased relationship to cycles of working chamber volume, to determine the net throughput of fluid through the machine. For example, EP 0 361 927 disclosed a method of controlling the net throughput of fluid through a multi-chamber pump by opening and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the pump and a low pressure manifold. Valves which regulate the flow of fluid between a low pressure manifold and a working chamber are referred to herein as low pressure valves. As a result, individual chambers are selectable by a controller, on a cycle by cycle basis, to carry out an active cycle and displace a predetermined fixed volume of fluid or to undergo an idle cycle with no net displacement of fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand.
EP 0 494 236 developed this principle and included electronically controllable poppet valves which regulate fluid communication between individual working chambers and a high pressure manifold, thereby facilitating the provision of a fluid working machine functioning as either a pump or a motor in alternative operating modes. Valves which regulate the flow of fluid between a high pressure manifold and a working chamber are referred to herein as high pressure valves. EP 1 537 333 introduced the possibility of part cycles, allowing individual cycles of individual working chambers to displace any of a plurality of different volumes of fluid to better match demand. GB 2430246 introduced a type of valve for regulating fluid communication between individual working chambers and a high pressure manifold, and a method of operating a machine with such a valve, that allowed the fluid working machine of EP 0 494 236 to develop a torque when stationary.
It is possible for these machines to fail if the timing of valve closure is not correct for the fluid pressure in the high pressure manifold. For example if, during a motoring cycle, a low pressure valve, such as a poppet valve, closes too late in the exhaust stroke to compress the trapped working fluid to at least the pressure of the high pressure manifold, then the high pressure valve of the respective working chamber will not open in preparation for drawing fluid from the high pressure manifold in a subsequent expansion stroke. Thus the motoring cycle is not possible and the machine malfunctions. In a second example, if the high pressure valve closes too late in the expansion stroke of a motoring cycle, this prevents the working chamber from sufficiently decompressing, thus preventing the respective low pressure valve from reopening to exhaust fluid from the working chamber and therefore causing fluid to be returned to the high pressure manifold on the compression stroke.
We have discovered that, in machines of this type, the properties of the working fluid change significantly in use, for example due to the ingress or absorption of air, water and other contaminants, operation at a wide range of temperatures and gradual deterioration over time. A particularly relevant and variable property is the fluid compressibility or bulk modulus. Also changes in fluid viscosity affect the rate of leakage of fluid out of the working chambers. Additionally the performance of the valves and other moving components can change over time as they wear, bed in or distort, or at different temperatures, causing them to individually act faster or slower at different times. Still further problems arise as fluid properties and valve performance are very difficult or expensive to measure during operation. In practice the operating fluid of a fluid working machine may be changed many times during its lifetime thereby changing the properties of the fluid, especially if fluid with different properties is selected on some occasions. Finally, it may be expensive to measure individual working chamber characteristics (such as leakage and valve closure times) during manufacture, and thus it may be desired to avoid calibrating the fluid working machine until it is used.
These factors conspire to reduce the accuracy of the flow into or out of the working machines, which is otherwise very accurately known. For example, closing the low pressure poppet valve at a selected phase relative to the cycle of working chamber volume would cause a smaller than expected volume to be pumped if the fluid compressibility or leakage was higher than expected.
Changes in the fluid properties can even cause the fluid working machine to fail in operation. For example an uncompensated increase in fluid compressibility or leakage would may mean that a low pressure poppet valve would close too late to sufficiently pressurise the working chamber and then open the high pressure valve in preparation for a motoring cycle. Thus the motoring cycle is not possible and the machine malfunctions. A second example is when an unexpected increase in compressibility or decrease in leakage prevents the working chamber from sufficiently decompressing at the end of the intake stroke of a motoring cycle, after the closure of the high pressure valve, thus preventing the low pressure valve from reopening.
The invention addresses the problem of enabling the fluid working machine to perform reliably and efficiently despite variations in the properties of working fluid which may arise in use, and also to methods of making relevant measurements.
In machines according to the prior art, the timing of closure of high and low pressure valves must always be conservative (i.e. early) to ensure that correct operation is achieved over the full range of fluid properties. However, this reduces the efficiency and capability of the machine because less fluid is displaced than would be the case were the timing less conservative. Also the closure of high and low pressure valves at times of higher flow creates more noise and could reduce the life of the valves, and can create undesirable torque and pressure ripple in the flow output of the fluid working machine. Thus, some aspects of the invention aim to increase the performance of a fluid working machine employing electronically controllable valves, operating over a range of fluid conditions or with component performance that varies over time, or to enable reduced specification electronically controllable valves to be employed than would otherwise be the case to obtain a fluid working machine with certain specified performance characteristics.